Nitric oxide (NO) and nitrogen dioxide (NO2): a state-of-knowledge review of their roles in, and consequences for, vegetation and ecosystems

The atmospheric pollutants nitric oxide (NO) and nitrogen dioxide (NO2) (collectively referred to as NOx) remain persistent constituents in the lower atmosphere. Here, we provide a state-of-knowledge overview of their sources, their biochemical and physiological roles in plants, and their consequences for ecosystems, exploring duality in their impacts. Ecosystem exposure to these pollutants is increasing in areas of the globe. Vegetative harm arises from oxidising and acidifying influences on biochemical and metabolic properties following stomatal uptake of NOx with conflicting information on whether NO and NO2 act differentially. Emergent general consequences include altered plant physiology, structure and function, with effects on vegetative growth, flowering and community composition e.g. loss of bryophytes and lichens. There is also evidence of negative effects on insect health and behaviour. Yet, recent evidence points to both NO and NO2 acting as plant signalling molecules and, in some ecosystems, providing a critical source of nutrients to sustain plant growth. The exogenous supply of NO from donor molecules, as well as endogenous NO, can mediate vegetation response to environmental stressors. Ecological responses are modulated by environmental factors (e.g. available nutrients) and other atmospheric constituents such as ozone, carbon dioxide and ammonia. We urgently need studies in realistic environments, including contemporary/future atmospheric conditions, to understand ecosystem consequences arising from atmospheric NOx exposure. Research needs to consider interactions among endogenous NO production in plants, stomatal uptake of NOx, and atmospheric NO to NO2 ratios to estimate risks to vegetation and ecosystems.